The invention pertains to a method for the measurement of the cavity volume of soft contact lenses and to the apparatus for performance of this method.
Until the present time, the following methods have been used for characterizing the shape of soft contact lenses (either hydrophilic or hydrophobic), necessary for their correct selection according to the examined shape of patient's eye:
1. A needle is shifted into the center of lens, which freely rests by its circumference on a planar base, as far as to the contact with the inner apex of lens. The sagittal depth of the lens determined in this way serves, together with the measured diameter of lens, as a rough measure of its total convexity.
2. Similarly, a needle is slided into the lens resting symmetrically on a circular edge of chosen diameter and the mean curvature of lens above this diameter is measured in this way.
3. A lens freely placed in an immersion liquid in a cell is measured by means of a projection or by means of photographic equipment.
4. A lens freed of surface water is measured by its reflection to determine the whole course of its inner or outer curves.
The methods 1, 2 and 4 are subjected to errors, arising from the deformation of soft lens by gravitational forces. Such measurement is entirely impossible with ultrathin lenses.
The method 3, where the immersed lens is not subjected to these deforming forces, is reliable for measurement of the lens shape, but it is very elaborate, because pictures must be made additionally precisely measured to evaluate the projection. In addition to this, strong reflections of tangentially illuminated surfaces occur at the side projection, thus making impossible the realiable measurement of the inner surface, which shape is even more important for application than the shape of the outer surface of the lens.